Various updates in preparation for Z3 version update

[marcoeilers]

We might want to update our used Z3 version from 4.8.7 to 4.16.0 soon. This version leads to some issues with the current version of Carbon, which I'm trying to address in this PR:

• Doing several asserts related to quantified permissions that we emit without triggers inside their own if (*) { ... assume false } block, to prevent them from being triggered in subsequent assertions.
• Dropping some Z3 flags
• Rewriting the axiom that states that for predicate P(x, y), P(x1, y1) == P(x2, y2) ==> x1 == x2 && y1 == y2 to one that uses getter functions for each parameter and therefore should not be multiplicative in the same way
• Some small changes to sequence axioms
• Introducing a timeout in the standard test suite